Automatic shifting power transmissions include a torque converter which provides torque multiplication at vehicle launch and also permits the vehicle to be held stationary and in gear without engine stall. The torque converter does reduce the overall efficiency of the powertrain during normal operation. To improve the efficiency, a torque converter clutch (TCC) is employed. The TCC is selectively engageable between the input impeller and output turbine of the torque converter to eliminate the fluid drive of the torque converter.
It is desirable to have the torque converter full of fluid when the engine is started to eliminate any lag or delay at vehicle launch. Thus, drain down of the torque converter to sump should be prevented. The drain down occurs when a normally open solenoid valve is connected in the control mechanism for the torque converter and clutch feed passage. The solenoid allows the torque converter to drain to sump over a period of time resulting in a delay between engine start and torque converter fill. Preventing or inhibiting the drain down phenomenon is accomplished, in current production transmissions, by installing a check valve and spring in the TCC release passage formed in the transmission input shaft. This shaft connects the turbine with the planetary gearing. This has generally been effective to prevent drain down. However, in some instances, drain down still occurs if the vehicle is idle for an extended period and the check valve and installation cost are high resulting in an increased cost of the transmission assembly. Another drawback to this solution is the flow restriction that the valve and spring introduce into the TCC release passage and the check valve can be inadvertently omitted at assembly requiring disassembly, repair and reassembly.